1. Field of the Invention
The present invention relates to a high zoom-ratio zoom lens system which is suitable as an imaging optical system for use mainly with an imaging sensor, the high zoom-ratio zoom lens system having a wide angle-of-view of a half angle-of-view of approximately 35 degrees. The present invention also relates to an electronic imaging apparatus which uses such a high zoom-ratio zoom lens system.
2. Description of Related Art
In recent years, there has been an increased need for digital cameras that have a high zoom-ratio zoom lens system including a wide angle-of-view range. In addition, further advancements in the miniaturization of the pixel pitch have been made, requiring the optical system to have a higher resolution. In an imaging lens system that is for use mainly in a compact camera, it is common for such a compact camera to achieve a slim design via the employment of a retractable barrel mechanism, in which the air-distances between lens groups are reduced at a lens barrel retracted state (accommodated state). However, when designing a zoom lens system which is suitable for such a retractable barrel mechanism, in addition to reducing the thickness of each lens group in a lens system, it is also necessary for the frontmost lens to be small in diameter, and furthermore, for this lens system to be designed with consideration of the mechanical restrictions in regard to the overall length thereof.
Zoom lens systems for use in high zoom-ratio compact cameras are known in the art; for example, such as those proposed in Japanese Unexamined Patent Publication Nos. 2006-23529 and 2006-308957; these zoom lens systems have relatively few lens elements and achieve miniaturization. However, in the zoom lens system disclosed in Japanese Unexamined Patent Publication No. 2006-23529, the zoom ratio is approximately 5:1 and the angle-of-view at the short focal length extremity is 61 degrees, and hence, does not achieve a sufficiently wide angle-of-view nor a sufficiently high zoom ratio. Furthermore, in the zoom lens system disclosed in Japanese Unexamined Patent Publication No. 2006-308957, a wide angle-of-view of 78 degrees at the short focal length extremity is achieved, however, the zoom ratio is within a range of approximately 4:1 through 7:1, which is still unsatisfactory. The zoom lens system disclosed in Japanese Unexamined Patent Publication No. 2008-112013 is relatively compact, and achieves both a wide angle-of-view and a high zoom ratio; however, a large amount of astigmatism and chromatic aberration occur, and hence, cannot attain the standard required in a zoom lens system that is suitable for high pixelization.